1. Field of the Invention
The present invention relates to image-forming optical systems using a reflecting surface that is decentered and has a power, for example, an image-forming optical system for use in cameras, video cameras, etc., and an image-forming optical system used in finder optical systems and so forth.
2. Discussion of Related Art
Recently, there have been proposed optical systems designed to be compact in size by giving a power to a reflecting surface and folding an optical path in the direction of the optical axis. In such optical systems, a prism or a mirror is mainly used as a member having a reflecting surface with a power. An optical system having a prism and an optical system having a mirror are largely different in characteristics from each other although these optical systems are the same in terms of the structure using a reflecting surface.
When a curvature (radius r of curvature) is given to a reflecting surface of a prism and to a reflecting surface of a mirror, the power of each of the reflecting surfaces is given by the paraxial power calculating equation as follows. The power of the reflecting surface of the prism is -2n/r in a case where the prism is filled therein with a medium having a refractive index n larger than 1, whereas the power of the reflecting surface of the mirror is -2/r. Thus, even when these reflecting surfaces have the same curvature, the powers are different from each other. Accordingly, the curvature required for the prism is 1/n of the curvature required for the mirror to obtain the same power. Therefore, the prism produces a smaller amount of aberration at the reflecting surface than in the case of the mirror. Thus, the prism is more favorable than the mirror in terms of performance. Moreover, the prism has two refracting surfaces, i.e. an entrance refracting surface and an exit refracting surface, in addition to a reflecting surface as a single member. Therefore, the prism is advantageous from the viewpoint of aberration correction in comparison to the mirror, which has only a reflecting surface as a single member. Furthermore, because the prism is filled with a medium having a refractive index larger than 1, it is possible to obtain a longer optical path length than in the case of the mirror, which is placed in the air. Accordingly, it is relatively easy with the prism to provide the required reflecting surface even when the focal length is short. In general, reflecting surfaces require a high degree of accuracy for assembly because decentration errors of reflecting surfaces cause the performance to be degraded to a considerable extent in comparison to refracting surfaces. In a case where an optical system is constructed by arranging a plurality of reflecting surfaces, the prism is more advantageous than the mirror because the prism enables a plurality of reflecting surfaces to be integrated into one unit so as to fix the relative positions and is therefore capable of preventing performance degradation due to assembling. Thus, the prism is superior to the mirror in many respects.
Meanwhile, when a surface with a power is placed at a tilt to the optical axis, rotationally asymmetric aberrations are produced. For example, if a rotationally asymmetric distortion occurs, a square object may become trapezoidal undesirably. Such rotationally asymmetric aberrations (hereinafter referred to as "decentration aberrations") are impossible to correct by a rotationally symmetric surface in theory. For this reason, rotationally asymmetric curved surfaces, e.g. anamorphic surfaces, are used in conventional prism optical systems.
Such prism optical systems include the disclosure of Japanese Patent Application Unexamined Publication (KOKAI) Number [hereinafter referred to as "JP(A)"] 8-313829. JP(A) 8-313829 discloses an ocular optical system comprising a prism in which there are two reflections, and a first transmitting surface and a second reflecting surface, as counted from the pupil side, are formed from the identical surface. In this optical system, all reflecting surfaces are rotationally asymmetric anamorphic surfaces.
Among the conventional prism optical systems using rotationally asymmetric curved surfaces, prism optical systems in which there are three reflections, in particular, are disclosed in JP(A) 9-33855, 9-73043 and 9-197336. These optical systems use spherical or anamorphic surfaces as reflecting surfaces.
JP(A) 9-33855 discloses an ocular optical system in which an optical axis thereof forms an optical path that makes one turn in the prism. A third reflecting surface and a first transmitting surface, as counted from the pupil side, are formed from the identical surface, and a first reflecting surface and a second transmitting surface, as counted from the pupil side, are formed from the identical surface. The prism optical system has only one reflecting surface that is independent of other transmitting and reflecting surfaces, i.e. the second reflecting surface. The direction in which light exits from the prism optical system is about 45 degrees oblique to the direction in which light enters the prism optical system.
JP(A) 9-73043 discloses an ocular optical system in which an optical axis thereof forms an M-shaped optical path. In Example 5 of JP(A) 9-73043, for instance, a second reflecting surface and a second transmitting surface, as counted from the pupil side, are formed from the identical surface. The prism optical system has only two surfaces that are independent of other transmitting and reflecting surfaces, i.e. a first reflecting surface and a third reflecting surface. In this example, the direction in which light exits from the prism optical system is opposite to the direction in which light enters the optical system. In JP(A) 9-197336, which has an arrangement similar to the above, a second reflecting surface, as counted from the pupil side, is formed from the identical surface with a first transmitting surface and a second transmitting surface.
These prior art prism optical systems suffer, however, from various problems as stated below.
In JP(A) 8-313829, the reflecting surfaces of the prism are given a power. However, because the prism optical system has only two reflecting surfaces, there is a limit in achieving a compact optical system while ensuring the required performance. If the aperture becomes large or the field angle becomes large, the optical system may fail to fulfill the required performance.
Accordingly, it is conceivable to increase the number of reflections so that aberration correction can be made even more effectively. However, a reduction in size and an increase in performance cannot simultaneously be attained in all the prior art prism optical systems in which there are three reflections, that is, the number of reflections is larger than that in the above-described prior art prism optical systems by one.
In JP(A) 9-33855, the optical path is arranged to turn in the prism. Therefore, a reduction in size of the prism can be attained effectively by folding the optical path. However, as the light beam becomes large, it is difficult to form two transmitting surfaces and three reflecting surfaces by using independent surfaces, respectively, owing to the structure thereof. Therefore, it is inevitably necessary to form the first transmitting surface and the third reflecting surface from the identical surface and to form the second transmitting surface and the first reflecting surface from the identical surface. Consequently, the angle of reflection at each of the first and third reflecting surfaces needs to satisfy the condition for total reflection. Therefore, aberration correction cannot satisfactorily be effected. In addition, because the angle of reflection is limited at two of the three reflecting surfaces, there is almost no freedom for the exit direction with respect to the entrance direction. Therefore, considering placement of another member, there are cases where it is impossible to achieve a reduction in size of the prism optical system.
In JP(A) 9-73043 and 9-197336, the prism optical system has an M-shaped optical path. Therefore, the second reflecting surface is likely to overlap the effective portion of a light beam passing through either or both of the first and second transmitting surfaces. Accordingly, the second reflecting surface unavoidably needs to be formed with the identical surface with the first and second transmitting surfaces. For this reason, the angle of reflection at the second reflecting surface needs to be equal to or larger than the angle for total reflection. Consequently, satisfactory aberration correction cannot be effected. In addition, because the exit direction is nearly parallel to the entrance direction, if the back focus is increased, or if another optical system is connected to the prism optical system, the resulting optical system becomes undesirably large in size in the entrance direction. Therefore, there are cases where it is impossible to achieve a reduction in size of the optical system.
Thus, all the prior art prism optical systems involve problems in terms of performance or size. There has heretofore been no compact and high-performance prism optical system that satisfies the demand for an improvement in performance and the demand for a reduction in size at the same time.